The 24-year-old was one of the stars of his country's FIFA World Cup campaign, contributing three goals as Ecuador finished third in Group E, and arrives at Upton Park on a five-year-deal.
West Ham confirmed their capture of Valencia - sealed for an undisclosed fee - with a statement on their official website on Thursday.
"West Ham United are delighted to announce that exciting Ecuador international forward Enner Valencia will become the club's fifth summer signing, subject to a successful work permit hearing," the statement read.
Liga MX outfit Pachuca only signed Valencia from Emelec in January, and he has scored 18 goals in 23 league matches since.
Valencia had previously helped Emelec to a first Serie A title in Ecuador for 11 years in 2013, and his domestic form saw him earn his international debut with Ecuador in February 2012.
The deal will mark West Ham's fifth signing of the transfer window, and their second forward addition following the purchase of Mauro Zarate from Velez Sarsfield.
Manager Sam Allardyce has been keen to bolster his attacking options after West Ham managed just 40 goals in the Premier League last term.
Former West Ham striker Teddy Sheringham has also been drafted in during the close-season to work as an attacking coach at Upton Park.
West Ham kick off their Premier League campaign against Tottenham on August 16, when Valencia could make his debut in the English top flight.
